Welcome to the dark world of torque converters. When it comes to automatic-equipped street machines and drag-race cars, the torque converter is a welded-up magic box that few enthusiasts know much about. Sure, there's all kind of talk about stall speed, slippage, and lockup features, but some gearheads secretly believe there's as much voodoo to a torque converter as there is science. To serve up a little enlightenment, let's take a spin through the torque-converter world and make some sense out of all the superstition.
The first automatic transmissions employed what was called a fluid coupling to connect the crankshaft with the transmission. These early units operated on the same principal of using one electric fan to drive another. The drive fan moves air across the blades of the driven fan (which is turned off). This slowly turns the blades of the driven fan in the same direction. This is how the first fluid couplings were designed, but they were hideously inefficient because of high slippage. In this design, the two basic components in the fluid coupling are the impeller blades (the drive side), which are connected to the engine, and the turbine blades (the driven member), which are splined to the input shaft of the transmission.
Automatics have become virtually standard in fast street cars on the dragstrip. The only way to put power to the ground with an automatic is with a properly sized converter that best matches your engine's power curve.
It took the addition of a small device called the stator to increase the efficiency of the fluid coupling and transform it into a torque converter. The stator redirects fluid from the turbine to the impeller, radically increasing the force of the fluid entering the impeller and multiplying the torque input from the engine. Basically, the fluid exits the center of the turbine and is redirected by the stator back into the impeller. This is a very slick trick and can be worth up to 2.5 times engine torque! This means that at a 2:1 torque multiplication ratio, an engine that is making 300 lb-ft of torque at a stall speed of 2,500 rpm will actually apply 600 lb-ft of torque to the input shaft of the transmission as the car leaves the starting line.
This sounds really great, and it is. But the catch is that this torque multiplication doesn't last very long. The multiplication factor is based on the speed difference between the turbine and impeller. At stall speed, the impeller is spinning at engine rpm and the turbine is stopped. This creates the greatest speed difference between the turbine and the impeller and therefore the maximum torque multiplication. Once the brakes (or the transbrake) is released and the vehicle begins to move, the turbine spins up and the speed difference between the two diminishes. Generally, by the time you reach the 60-foot mark on the dragstrip, torque multiplication is effectively eliminated and the turbine and impeller speeds are close to achieving what converter manufacturers call coupling speed.
Street/strip torque converters range in diameter from 10 to 13 inches. Larger-diameter converters are the most efficient but heavier. Companies like B&M and others also build "tight" small-diameter converters, especially for nitrous applications.
Achieving a true 1:1 speed relationship between the turbine and the impeller is not possible with a non-lockup torque converter since it is still a fluid coupling. Most street torque converters tend to operate in the 3 to 5 percent slippage range, but this can go as high as 8 percent. A 5 percent slippage factor means that if the impeller is spinning 3,000 rpm, the turbine is only spinning 2,850, or 5 percent slower. This slippage is part of what determines the stall speed of the converter. By changing the angle of the impeller blades, the torque-converter manufacturer can create a converter that generates a given stall speed with a certain torque multiplication.
A manufacturer's torque-converter rating is generally expressed as an rpm range. It's not more specific because stall speed is highly dependent upon engine torque. Stall speed is measured in the car by applying the brakes and revving the engine in First gear against the brakes. Most street cars cannot keep the rear tires from spinning if you add too much throttle, and this tends to limit the stall speed.
For example, let's say your 355ci small-block makes 230 lb-ft of torque at 2,500 rpm and you bolt in a B&M Holeshot 2000 converter that creates a 2,000-stall speed (this is actually the torque standard that B&M uses to establish stall speed for its street converters). Now let's bolt the same Holeshot 2000 converter behind a larger, 400ci small-block that makes 400 lb-ft of torque at 2,400 rpm. The same converter is now going to exhibit a much higher stall speed, perhaps 2,300 to 2,400 rpm, because the engine is cranking out 170 more lb-ft of torque. So as you can see, stall speed is not an exact science due to multiple factors that affect the converter. Furthermore, if you were to add a transbrake that locks the trans in two gears at once, stall speed will generally increase another 100 to as much as 300 rpm depending upon the application.
This B&M cutaway illustrates the three major components in a torque converter: the impeller (A), turbine (B), and stator (C). The stator includes an overrunning clutch that allows it to freewheel once the turbine and impeller reach a coupling speed. The copper-colored material on the fins is the result of the furnace-brazing process that increases the strength of the converter. | 
Weight is a big factor in performance, especially when we're talking about lockup-style converters. Lockup converters are more efficient, but they also weigh more. The lockup feature on a 245mm (9.5-inch) diameter B&M lockup converter will add 4.5 pounds to the converter's heft. |
This sectioned B&M lockup converter reveals the solid plate (arrow) that mounts the thin band material that "locks" the engine flywheel to the input shaft of the transmission. These small bands are only intended to handle light torque loads for cruising, not for wide-open throttle. | |